1. Field of the Invention:
The present invention relates to an improvement of a duty control solenoid valve, more particularly, to a solenoid valve capable of obtaining excellent flow control characteristic in all ranges of flow, i.e. from the small quantity of flow to large quantity of flow.
2. Prior Art:
A flow control solenoid valve having a plunger operable by an electromagnetic coil and a valve port which is opened or closed by the plunger wherein the opening time of the valve port is controlled to control the quantity of flow, a so-called duty control solenoid, has been used in various fields. The solenoid valve is hereinafter described with reference to a known solenoid valve which is provided in a purge line in an evaporative emission control system of a fuel tank of an automobile but the solenoid valve of the invention can be applied to a general purpose flow control solenoid valve.
The prior art solenoid valve is structured as illustrated in FIG. 4.
There is provided a plunger 3 which is operated toward a stator 2 by an electromagnetic attraction of an electromagnetic coil 1. A valve element 4 is provided on the plunger 3. There is provided a return spring 6 for urging the plunger 3 toward a valve port 5. The electromagnetic attraction of the electromagnetic coil 1 and the resiliency of the return spring 6 apply alternately to the plunger 3 whereby the valve port 5 is opened or closed by the valve element 4 in a given cycle. A flow control nozzle 8 is provided in a fluid passage 7 through which fluid flows.
According to the prior art solenoid valve, the maximum quantity of flow is determined by an opening area of the flow control nozzle 8 supposing that the pressure is constant. This is explained with reference to FIG. 5 showing the flow control characteristic. In the maximum quantity of flow Q1, the duty ratio is 100% where the valve element 4 always opens the valve port 5. The maximum quantity of flow is determined by the opening area of the flow control nozzle 8. Theoretically, the quantity of flow Q1 is proportional to the duty ratio and the flow control characteristic line is linear in FIG. 5.
There is the following problems in the prior art solenoid valve.
If the solenoid valve is used in the purge line of the evaporative emission control system, it is used in the range from the small quantity to large quantity of flow and a precise flow control is required in the small quantity of flow. However, in the prior art solenoid valve provided with the flow control nozzle having a constant opening area, it is difficult to precisely control the small quantity of flow.
This is described more in detail. If the flow control is performed in the range from the small quantity to large quantity of flow, the opening area of the flow control nozzle 8 is so large that the solenoid valve obtains a large quantity of fluid, namely, a large quantity of fluid is introduced in the solenoid valve through the flow control nozzle 8. However, such a flow control nozzle having the large opening area is inferior in its flow control characteristic in the small quantity of flow so that it can not perform the precise flow control in case of small quantity of flow.
On the other hand, if the precise flow control is performed in the small quantity of flow, the opening area of the nozzle 8 must be small, which however can not obtain a large quantity of fluid.
Accordingly, the prior art solenoid valve has been used by permitting the opening area of the flow control nozzle 8 to be large while sacrificing the flow control characteristic in the small quantity of flow.